a) Field of the Invention
The present invention relates to a zoom image pickup optical system which can be attached to eyepieces of endoscopes.
b) Description of the Prior Art
TV cameras and film cameras are often attached to eyepieces of endoscopes for diagnoses and recording. In recent days, in particular, compact TV cameras using solid-state image pickup devices such as CCDs are being increasingly adopted to project images formed with endoscopes onto TV monitors for diagnoses and medical treatments.
Since the image pickup devices used in the TV cameras are now made to be more compact and to have picture elements arranged at high densities due to recent progress made in the semiconductor technology, optical systems which are to be used in combination with the image pickup devices must have sufficiently high optical performance.
FIG. 1 shows a system for projecting an image of an object formed with an endoscope onto a TV monitor in which an adaptor 3 and a TV camera 5 are attached to an eyepiece 2 of an endoscope 1.
In the image projecting system having the configuration described above, an image of the object formed with the eyepiece 2 disposed in the endoscope 1 is imaged through an image pickup optical system 4 disposed in the adaptor 3 onto an image pickup device 6 disposed in the TV camera.
For practical use of the image projecting system, several kinds of adaptors having different magnifications are prepared and selectively used dependently on kinds, purposes, etc. of endoscopes to be combined. Accordingly, this system requires a large number of adaptors, are highly priced and obliges a user to exchange the adaptors when he desires to change a size of an observed location by switching magnifications in the course of use, thereby hardly permitting exchange of adaptors, for example, during a surgical operation. Further, it is actually impossible, by exchange of the adaptors, to control an image to a size desired by the user, for example a surgical operator, and he is obliged to find a certain point of compromise.
For correcting such a defect, it is often carried out to use a zoom optical system in the adaptor or a similar section for freely changing a size of an image of a location which is being observed.
Eyepieces of a specific kind of endoscope are mostly configured so as to have imaging points at a certain definite distance or a definite diopter and be set at a definite structural location when attached to adaptors.
When an optical system is combined with endoscopes which have a definite diopter, object points remain unchanged since the endoscopes have the definite diopter. Accordingly, the optical system, even if it is a zoom optical system, may not use a focusing mechanism as an optical system disclosed by Japanese Patent Kokai Publication No. Hei 1-128031. In the case of non-flexible endoscopes or the similar instruments, however, diopters of eyepieces may be different dependently on kinds of scopes or certain ordinary endoscopes use eyepieces having different diopters and diopters are actually changed dependently on object points.
When diopters are to be different dependently on endoscopes to be combined with a zoom optical system or objects to be observed, it is necessary to dispose a focusing mechanism in the zoom optical system. As a method to focus the zoom optical system, it is conceivable to move the zoom optical system as a whole in a direction along an optical axis. When a zoom optical system is moved as a whole for focusing as in an image pickup optical system disclosed by U.S. Pat. No. 4,781,445, for example, a focal length of the optical system as a whole is varied by zooming and a diopter of an endoscope is varied, whereby a focusing amount required for diopter adjustment, or moving distance of the optical system, is varied dependently on magnifications. When a focal length at a tele position is twice as long as that at a wide position, for example, a lens system must be moved for a distance four times as long for the same diopter adjustment. Accordingly, it is required to reserve wide spaces before and after the optical system, thereby enlarging an adapter as a whole. Further, a change of a magnification of the zoom optical system results in a variation of an amount of diopter adjustment, thereby making it impossible to provide a scale for indicating ranges of diopter adjustments. As a result, the image pickup system cannot indicate locations at which the zoom optical system is brought into focus even when the optical system is combined with an endoscope having a known diopter, thereby obliging users to focus the optical system while actually observing an image.
Since an adaptor is attached to an endoscope and operated integrally therewith, it is important to configure the adaptor to be as compact and light as possible for facilitating its manipulation or lessening burdens imposed on users or surgical operators.